Neuropathic pain and nociceptive pain differ in their etiology, pathophysiology, diagnosis and treatment. Nociceptive pain occurs in response to the activation of a specific subset of peripheral sensory neurons, the nociceptors. It is generally acute (with the exception of arthritic pain), self-limiting and serves a protective biological function by acting as a warning of on-going tissue damage. It is typically well localized and often has an aching or throbbing quality. Examples of nociceptive pain include post-operative pain, sprains, bone fractures, burns, bumps, bruises, inflammation (from an infection or arthritic disorder), obstructions and myofascial pain. Nociceptive pain can usually be treated with opioids and non-steroidal anti-inflammatory drugs (NSAIDS).
Neuropathic pain is a common type of chronic, non-malignant, pain, which is the result of an injury or malfunction in the peripheral or central nervous system and serves no protective biological function. It is estimated to affect more than 1.6 million people in the U.S. population. Neuropathic pain has many different etiologies, and may occur, for example, due to trauma, diabetes, infection with herpes zoster (shingles), HIV/AIDS, late-stage cancer, amputation (including mastectomy), carpal tunnel syndrome, chronic alcohol use, exposure to radiation, and as an unintended side-effect of neurotoxic treatment agents, such as certain anti-HIV and chemotherapeutic drugs.
In contrast to nociceptive pain, neuropathic pain is frequently described as “burning”, “electric”, “tingling” or “shooting” in nature. It is often characterized by chronic allodynia (defined as pain resulting from a stimulus that does not ordinarily elicit a painful response, such as light touch) and hyperalgesia (defined as an increased sensitivity to a normally painful stimulus), and may persist for months or years beyond the apparent healing of any damaged tissues.
Neuropathic pain is difficult to treat. Analgesic drugs that are effective against normal pain (e.g., opioid narcotics and non-steroidal anti-inflammatory drugs) are rarely effective against neuropathic pain. Similarly, drugs that have activity in neuropathic pain are not usually effective against nociceptive pain. The standard drugs that have been used to treat neuropathic pain appear to often act selectively to relieve certain symptoms but not others in a given patient (for example, relief of allodynia, but not hyperalgesia). For this reason, it has been suggested that successful therapy may require the use of multiple different combinations of drugs and individualized therapy (see, for example, Bennett, Hosp. Pract. (Off Ed). 33:95-98, 1998). Treatment agents typically employed in the management of neuropathic pain include tricylic antidepressants (for example, amitriptyline, imipramine, desimipramine and clomipramine), systemic local anesthetics, and anti-convulsants (such as phenyloin, carbamazepine, valproic acid, clonazepam and gabapentin).
Many anti-convulsants originally developed for the treatment of epilepsy and other seizure disorders have found application in the treatment of non-epileptic conditions, including neuropathic pain, mood disorders (such as bipolar affective disorder), and schizophrenia (for a review of the use of anti-epileptic drugs in the treatment of non-epileptic conditions, see Rogawski and Loscher, Nat. Medicine, 10:685-692, 2004). It has thus been suggested that epilepsy, neuropathic pain and affective disorders have a common pathophysiological mechanism (Rogawski & Loscher, ibid; Ruscheweyh & Sandkuhler, Pain 105:327-338, 2003), namely a pathological increase in neuronal excitability, with a corresponding inappropriately high frequency of spontaneous firing of neurons. However, only some, and not all, antiepileptic drugs are effective in treating neuropathic pain, and furthermore such antiepileptic drugs are only effective in certain subsets of patients with neuropathic pain (McCleane, Expert. Opin. Pharmacother. 5:1299-1312, 2004).
Epilepsy is characterized by abnormal discharges of cerebral neurons and is typically manifested as various types of seizures. Epileptiform activity is identified with spontaneously occurring synchronized discharges of neuronal populations that can be measured using electrophysiological techniques. This synchronized activity, which distinguishes epileptiform from non-epileptiform activity, is referred to as “hypersynchronization” because it describes the state in which individual neurons become increasingly likely to discharge in a time-locked manner with one another. Hypersynchronized activity is typically induced in experimental models of epilepsy by either increasing excitatory or decreasing inhibitory synaptic currents, and it was therefore assumed that hyperexcitability per se was the defining feature involved in the generation and maintenance of epileptiform activity. Similarly, neuropathic pain was believed to involve conversion of neurons involved in pain transmission from a state of normal sensitivity to one of hypersensitivity (Costigan & Woolf, Jnl. Pain 1:35-44, 2000). The focus on developing treatments for both epilepsy and neuropathic pain has thus been on suppressing neuronal hyperexcitability by either: (a) suppressing action potential generation; (b) increasing inhibitory synaptic transmission; or (c) decreasing excitatory synaptic transmission. However, it has been shown that hypersychronous epileptiform activity can be dissociated from hyperexcitability and that the cation chloride cotransport inhibitor furosemide reversibly blocked synchronized discharges without reducing hyperexcited synaptic responses (Hochman et al. Science 270:99-102, 1995).
Both abnormal expression of sodium channel genes (Waxman, Pain 6:S133-140, 1999; Waxman et al. Proc. Natl. Acad. Sci USA 96:7635-7639, 1999) and pacemaker channels (Chaplan et al. J. Neurosci. 23:1169-1178, 2003) are believed to play a role in the molecular basis of neuropathic pain.
The cation-chloride co-transporters (CCCs) are important regulators of neuronal chloride concentration that are believed to influence cell-to-cell communication, and various aspects of neuronal development, plasticity and trauma. The CCC gene family consists of three broad groups: Na+—Cl− co-transporters (NCCs), K+—Cl− co-transporters (KCCs) and Na+—K+-2Cl− co-transporters (NKCCs). Two NKCC isoforms have been identified: NKCC1 is found in a wide variety of secretory epithelia and non-epithelial cells, whereas NKCC2 is principally expressed in the kidney. For a review of NKCC1 structure, function and regulation see, Haas and Forbush, Annu. Rev. Physiol. 62:515-534, 2000. Randall et al. have identified two splice variants of the Slc12a2 gene that encodes NKCC1, referred to as NKCC1a and NKCC1b (Am. J. Physiol. 273 (Cell Physiol. 42):C1267-1277, 1997). The NKCC1 a gene has 27 exons, while the splice variant NKCC1b lacks exon 21. The NKCC1b splice variant is expressed primarily in the brain. NKCC1b is believed to be more than 10% more active than NKCC1a, although it is proportionally present in a much smaller amount in the brain than is NKCC1a. It has been suggested that differential splicing of the NKCC1 transcript may play a regulatory role in human tissues (Vibat et al. Anal. Biochem. 298:218-230, 2001). Na—K—Cl co-transport in all cell and tissues is inhibited by loop diuretics, including furosemide, bumetanide and benzmetanide.
Na—K-2Cl co-transporter knock-out mice have been shown to have impaired nociception phenotypes as well as abnormal gait and locomotion (Sung et al. Jnl. Neurosci. 20:7531-7538, 2000). Delpire and Mount have suggested that NKCC1 may be involved in pain perception (Ann. Rev. Physiol. 64:803-843, 2002). Laird et al. recently described studies demonstrating reduced stroking hyperalgesia in NKCC1 knock-out mice compared to wild-type and heterozygous mice (Neurosci. Letts. 361:200-203, 2004). However, in this acute pain model no difference in punctuate hyperalgesia was observed between the three groups of mice. Morales-Aza et al. have suggested that, in arthritis, altered expression of NKCC1 and the K—Cl co-transporter KCC2 may contribute to the control of spinal cord excitability and may thus represent therapeutic targets for the treatment of inflammatory pain (Neurobiol. Dis. 17:62-69, 2004). Granados-Soto et al. have described studies in rats in which formalin-induced nociception was reduced by administration of the NKCC inhibitors bumetanide, furosemide or piretanide (Pain 114:231-238, 2005). While the formalin-induced acute pain model is extensively used, it is believed to have little relevance to chronic pain conditions (Walker et al. Mol. Med. Today 5:319-321, 1999). Co-treatment of brain damage induced by episodic alcohol exposure with an NMDA receptor antagonist, non-NMDA receptor and Ca2+ channel antagonists together with furosemide has been shown to reduce alcohol-dependent cerebrocortical damage by 75-85%, while preventing brain hydration and electrolyte elevations (Collins et al, FASEB J., 12:221-230, 1998). The authors stated that the results suggest that furosemide and related agents might be useful as neuroprotective agents in alcohol abuse. Willis et al. have published studies indicating that nedocromil sodium, furosemide and bumetanide inhibit sensory nerve activation to reduce the itch and flare responses induced by histamine in human skin in vivo. Espinosa et al. and Ahmad et al. have previously suggested that furosemide might be useful in the treatment of certain types of epilepsy (Medicina Espanola 61:280-281, 1969; and Brit. J. Clin. Pharmacol. 3:621-625, 1976).
As with epilepsy, the focus of pharmacological intervention in neuropathic pain has been on reducing neuronal hyperexcitability. Most agents currently used to treat neuropathic pain target synaptic activity in excitatory pathways by, for example, modulating the release or activity of excitatory neurotransmitters, potentiating inhibitory pathways, blocking ion channels involved in impulse generation, and/or acting as membrane stabilizers. Conventional agents and therapeutic approaches for the treatment of neuropathic pain and neuropsychiatric disorders thus reduce neuronal excitability and inhibit synaptic firing. One serious drawback of these therapies is that they are nonselective and exert their actions on both normal and abnormal neuronal populations. This leads to negative and unintended side effects, which may affect normal CNS functions, such as cognition, learning and memory, and produce adverse physiological and psychological effects in the treated patient. Common side effects include over-sedation, dizziness, loss of memory and liver damage. There is therefore a continuing need for methods and compositions for treating neuronal disorders that disrupt hypersynchronized neuronal activity without diminishing the neuronal excitability and spontaneous synchronization required for normal functioning of the peripheral and central nervous systems.